Motivation

Ambient occlusion is a lighting technique used to give models a global illumination-like effect, as if it were lit from the entire hemisphere (rather than a point light). A set of visibility samples are collected from the hemisphere above a point, and a scalar “ambient occlusion” value is computed based on the percentage of unoccluded samples. For a nice overview of ambient occlusion, check out Steve Hill’s article in Gamastura. Some other very good ambient occlusion tutorials have been written by Zhang Jian, Bob Moyer, and Andrew Whitehurst.

Approaches

There are two basic approaches for computing ambient occlusion. The first is ray tracing; simply shoot out rays in uniform pattern across the hemisphere. The percentage of rays that hit geometry is divided by the total number of rays to get a scalar value. This could also be done with hardware rendering by placing a camera at every triangle and counting the percentage of the hemisphere that is visible. Since this requires computing occlusion at every triangle, this approach is in general O(N^2), although with efficiency structures (such as bounding volume hierarchies) it can be reduced to O(NlogN).

The second approach is an “outside-looking-in” approach. The model is rendered from a set of M random points on the sphere surrounding the model. The occlusion information is stored for each viewpoint, and averaged. The complexity of this approach is O(N*M), or O(MlogN) with efficiency structures. Since M is generally much smaller than N, this makes it interesting for realtime work. Since this is the approach I am interested in, let me explain the algorithm in more detail.

goto full Assorted Notes :

http://www.cs.unc.edu/~coombe/research/ao/

Global illumination is a very popular method for lighting three dimensional scenes. Unfortunately, most of the methods are very slow and requires lots of ressources. Ambient Occlusion is a method for simulating global illumination, while keeping a respectable ratio between results and computing times.

As well as anything related to raytracing with Blender, because of the supplementary computing time required by your computer, raytracing is only an option that you should feel free to activate and deactivate. This is done by the mean of the Scene menu (F10 key). The Render tab shows a button labeled Ray you will have to activate in order to use raytracing in your pictures.

goto Tutorial :

http://feeblemind.tuxfamily.org/dotclear/index.php/2005/01/16/10-didacticiel-locclusion-ambiante—tutorial-ambient-occlusion

In 3dsmax7 Mental Ray 3.3 gained a new shader called “Ambient / Reflective Occlusion” what can this be used for? Well, traditionally within Mental Ray GI has been handled photon mapping and Final Gather. In this case a GI effect can be generated via a shader, and in this way this system opens up some flexibility in how you generate your GI.

So what does this shader do? Quite simply it shoots out rays from the pixel that is being shaded in a hemisphere based on the normal of the pixel. It then traces these rays to the scene bounds or to a fixed distance defined in the shader and if it hits a piece of geometry the ray is flagged as occluded. Mental Ray then works out the average number of rays that were occluded and uses this to work out a blend colour between the BRIGHT and DARK values setup in your shader. On the right is a diagram of a typical pixel being shaded.

In the example shown you can see that our shader has thrown out a total of 5 rays. If the BRIGHT colour were white, and the DARK colour Black, we would have two DARK rays and three LIGHT rays. So our pixel would be shaded as 40% occluded, or in other words a grey of 40% black would be used. Due to the way Monti-Carlo sampling works, 5 rays would not give us a very good average, pretty much in the same way as a sampling of 5 people in a survey would not give a very good average opinion of Mr man in the street. So for a better quality render you would expect to use 24+ samples.

goto Tut0rial :

http://www.christopher-thomas.net/pages/free_tutorials/tut_ambient_occlusion_shader/ct_tut_mentalray_ambientocclusion_shader.htm